The present invention relates to an apparatus for manufacturing spring units suitable for forming mattresses or box spring mattresses.
Spring units each comprising a wire whose both ends are formed as torsion bar springs have been well-known as the spring units used in forming mattresses or box spring mattresses. It has been pointed out, however, that the mattress formed by torsion bar springs has the following problem. Because the torsion bar spring has small deformation ability relative to the compressed load applied, when incorporated into a mattress it has the necessary hardness to reliably support a person. However, the mattress is likely to be strained permanently when a load is repeatedly applied to it, as the spring unit provided with torsion bar springs quickly loses its elasticity.
In order to solve this problem, therefore, a mattress which is formed by spring units each comprising a wire whose both ends are formed as coil springs is proposed by U.S. Ser. No. 493,330 filed May 10, 1983 and its corresponding EPC Application No. 83105291.5 filed May 27, 1983.
When the spring portion is formed by a coil spring, it has better fatique characteristics, as compared with the torsion bar spring portion, and the elasticity of the spring unit can be thus kept good over a long time period. However, no apparatus has yet been developed for mass producing spring units comprising a linear portion whose both ends are formed as continuous coil springs. This is the reason why it has not been possible to provide box spring mattresses having spring units with coil springs.